The role of fuel and environmental conditions on the amount and composition of primary, fresh, and aged aerosol emissions originating from diesel- and gasoline-operated auxiliary heaters of passenger cars

Abstract. Fuel-operated auxiliary heaters (AHs) are potentially significant additional sources of particle- and gas-phase pollution from vehicles, but information on their emissions is scarce. In particular, an understanding of secondary aerosol formation originating from AH exhaust is lacking. In this study, we measured the gas and particle emissions, including secondary emissions, of diesel- and gasoline-operated AHs used in passenger cars. Investigation revealed the importance of peak emissions during start and shutdown events of the heaters and differences between emissions of gasoline- and diesel-fuelled AHs: gasoline-operated AHs also produced particles under steady-state operating conditions, while their diesel counterparts did not. Furthermore, ambient air temperature was observed to impact the emission profiles, with, for example, higher nitrogen oxide (NOx) and particle mass emissions but lower particle number (PN) emissions observed in outdoor (−19 to −7 °C) measurements compared to laboratory measurements (+25 °C). However, further quantification is necessary to fully connect the temperature-related effects and AH emissions. Our findings highlight the importance of also characterizing the atmospherically aged aerosols, specifically secondary organic aerosol (SOA) formation, which was simulated here both by an environmental chamber and by an oxidation flow reactor (OFR). The particle mass in photochemically aged aerosols surpassed the fresh exhaust particulate mass emissions by 1 to 3 orders of magnitude, with the increase depending mainly on fuel, combustion conditions, and ageing methods. Further research into formation pathways of secondary aerosols from precursors is still needed, along with the quantification of vehicle AH emissions at the fleet level, to enable the estimation of atmospheric and air quality effects of AH usage.